Resin acid
Resin acids are protectants and wood preservatives that are produced by parenchymatous epithelial cells that surround the resin ducts in trees from temperate coniferous forests. The resin acids are formed when two- and three-carbon molecules couple with isoprene building units to form mono-, sesqui-, and diterpene structures. Resin acids have two functional groups: carboxyl group and double bonds. Nearly all have the same basic skeleton: a 3-ring fused system with the empirical formula C19H29COOH.
Pines contain numerous vertical and radial resin ducts scattered throughout the entire wood. The accumulation of resin in the heartwood and resin ducts causes a maximum concentration in the base of the older trees. Resin in the sapwood, however, is less at the base of the tree and increases with height.
Natural resins are water-insoluble mixtures of compounds, many of which have a hydroaromatic structure. Mixtures of isomeric carboxylic acids, such as abietic and pimaric acids, which occur in rosin in nature in solvent-free form, in the form of tree sap or wood rosin such as pine oleoresin, where they are dissolved in terpenic hydrocarbons. They can also be present as fossil coal or copal resins, in old pine tree stumps, etc.
Commercially, the manufacture of wood pulp grade chemical cellulose using the kraft chemical pulping processes releases these resin acid compounds. The Kraft process is conducted under strongly basic conditions of sodium hydroxide, sodium sulphide and sodium hydrosulphide which neutralizes these resin acids, converting them to their respective sodium salts, sodium abietate, ((CH3)4C15H17COONa) sodium pimarate ((CH3)3(CH2)C15H23COONa) and so on. In this form, the sodium salts are insoluble and, being of lower density that the spent pulping process liquor, float to the surface of storage vessels during the process of concentration, as a somewhat gelatinous pasty fluid called kraft soap, or resin soap.
Kraft soap can be reneutralized in the presence of concentrated sulphuric acid to restore the acidic forms abietic acid, palmiric acid and their isomers which form the resin acid component of a pulping byproduct called tall oil. Other major components include fatty acids and unsaponifiable sterols.
Resin acids, because of the same protectant nature they provide in the trees where they originate, also impose toxic implications on the effluent treatment facilities in pulp manufacturing plants. Furthermore, any residual resin acids that pass the treatment facilities add toxicity to the stream discharged to the receiving waters.